Summary of Work: Magnetic fields associated with the use and transmission of electricity have been reported to depress nocturnal levels of melatonin in rodents, especially hamsters. In vitro models have shown that high melatonin levels can slow the growth of human breast cancer cells thus providing some support for the association between children living in homes close to power lines and increased cancer rates. Since exposure to light will depress melatonin levels more effectively than magnetic fields, this model will use light to depress melatonin and explore the effect on breast epithelial cells. Since breast cancer continues to increase in women, is one of the leading causes of death and since the factor(s) leading to the increasing rates is not known, this project is of great public health significance.